The long-term goals of this work are to elucidate the sequence of cellular and molecular events occurring during vertebrate skeletal muscle development. Permanent clonal lines of mouse myoblasts, drug resistant variants from these lines and somatic cell hybrids will be used to analyze sequential events which occur as proliferating myoblasts become committed to terminal muscle differentiation. Particular attention will be focused on the roles of specific mitogens in controlling myoblast proliferation and on whether myoblasts, exhibit stage-specific mitogen responsiveness. A related phase of the project will involve quantitation of biochemical changes occurring in synchronously differentiating muscle cultures. A second aspect of the study involves analysis of somatic cell hybrids between mouse and human cells of myogenic and non-myogenic types. The goals are: (a) to determine the chromosomal linkage of human muscle-specific genes; and (b) to determine the extent to which combination with non-myogenic cells affects expression of muscle-specific genes. A third aspect of the study involves positional analysis of muscle-colony-forming cells (MCF cells) from specific chick, mouse, and human leg bud regions. Regional differences will be quantitated and discrete MCF cells will be isolated and subjected to comparative studies.